Fixed #15241 -- Updated the upgrading notes for generic views to reflect...

Fixed #15241 -- Updated the upgrading notes for generic views to reflect recent changes in class-based views. Thanks to Jonney for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15479 bcc190cf-cafb-0310-a4f2-bffc1f526a37